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Abstract

We present the first observation, to our knowledge, of lasing from a levitated, dye droplet. The levitated droplets are created by computer controlled pico-liter dispensing into one of the nodes of a standing ultrasonic wave (100 kHz), where the droplet is trapped. The free hanging droplet forms a high quality optical resonator. Our 750 nL lasing droplets consist of Rhodamine 6G dissolved in ethylene glycol, at a concentration of 0.02 M. The droplets are optically pumped at 532 nm light from a pulsed, frequency doubled Nd:YAG laser, and the dye laser emission is analyzed by a fixed grating spectrometer. With this setup we have achieved reproducible lasing spectra in the visible wavelength range from 610 nm to 650 nm. The levitated droplet technique has previously successfully been applied for a variety of bio-analytical applications at single cell level. In combination with the lasing droplets, the capability of this high precision setup has potential applications within highly sensitive intra-cavity absorbance detection.

Figures (3)

a) Photograph of a lasing levitated micro-droplet. b) Schematics of ultrasonic field with the micro-droplet being trapped at a node in the ultrasonic field. c) Schematics of whispering-gallery modes in a (2D) spherical cavity. d) Numerical example of a whispering-gallery mode in a (2D) spherical cavity.

Reproducible lasing spectra from dye doped micro-droplets. Each spectrum is obtained in a fixed setup by a well-controlled loading of an EG droplet with a Rh6G dye which is subsequently pumped above threshold, see Fig. 3(b). The spectra are averaged over three pump pulses.

a) Cavity output power for increasing and decreasing average pump power. Each spectrum is obtained in a fixed setup by pumping an EG droplet with a Rh6G dye. The pump power is first increased from zero up to level around 1000mW (dashed curves) and subsequently again lowered (solid curves). The spectra are averaged over three pump pulses. b) Cavity output power versus mean pump power. The dashed lines are guides to the eyes indicating a lasing threshold of around 500mW in the mean pump power.